1. Field of the Invention
The present invention relates to an image-sensing apparatus having a photoelectric conversion device that outputs a signal that is logarithmically proportional to the amount of incident light.
2. Description of the Prior Art
A conventional area sensor having photosensitive devices such as photodiodes outputs a signal that is linearly proportional to the brightness of the light incident on the photosensitive devices as shown at (a) in FIG. 15. When a subject having brightness distribution as shown at (b) in FIG. 15 is shot with such a linear-conversion-based area sensor (hereafter referred to as a “linear sensor”), no brightness data is obtained outside the roughly two-digit brightness range (dynamic range) within which the linear sensor can effectively perform image sensing (this brightness range will hereinafter be referred to as the “shootable brightness range”).
Accordingly, when the signal from this linear sensor is reproduced as an image on a display or the like, the displayed image suffers from flat blackness in low-brightness portions thereof and saturation in high-brightness portions thereof outside the shootable brightness range. It is possible to avoid flat blackness by shifting the shootable brightness range leftward or avoid saturation by shifting it rightward. However, this requires varying the aperture value or shutter speed of a camera, or the integral time for which to allow light in, and thus spoils ease of use.
On the other hand, in U.S. Pat. No. 5,241,575, the applicant of the present invention once proposed an area sensor (hereinafter referred to as a “LOG sensor”) provided with a light-sensing means that outputs a photocurrent in proportion to the amount of incident light, a MOS transistor to which the photocurrent is fed, and a bias means for biasing the MOS transistor in such a way that a subthreshold current flows therethrough, so that the photocurrent is converted logarithmically. This LOG sensor outputs a signal whose level is natural-logarithmically proportional to the amount of incident light as shown at (a) in FIG. 16, and thus offers a wide, specifically five- to six-digit, dynamic range. This permits, even in cases in which the brightness distribution tends to shift, the brightness distribution of a given subject to lie most probably within the shootable brightness range as shown at (b) in FIG. 16.
However, a typical subject has a two- to three-digit brightness range, and therefore, if it is shot with a LOG sensor that offers a five- to six-digit dynamic range, the shootable brightness range is too wide relative to the actual brightness distribution of the subject, and thus a region where no brightness data is available is left in a low-brightness or high-brightness portion of the shootable brightness range. Specifically, as shown at (a) in FIG. 16, with respect to the dynamic range DRa of the LOG sensor, the range DRb of the output that corresponds to the brightness distribution of the subject is considerably narrow. Thus, when the output signal of this LOG sensor is subjected to level conversion so as to be converted, for example, into an eight-bit digital signal as shown in FIG. 17 in order to display an image on an output device such as a display, if conversion is so performed that the dynamic range DRc of the output device is adapted to the dynamic range DRa of the LOG sensor and that the maximum and minimum output values of the LOG sensor correspond to the maximum and minimum output levels (255 and 0), respectively, of the output device, the output device receives a signal whose level varies only within a range DRd which is merely a portion of the range covered by the eight-bit digital signal. Thus, when the image of a subject having brightness distribution as described above is reproduced using such a signal, the black portions of the image, where the brightness is lowest, are reproduced as dark gray, and the white portions thereof, where the brightness is highest, are reproduced as light gray, making the entire image low-contrast.